With over 15 million past month users, marijuana is the most widely used illicit substance in the United States. This is concerning because marijuana use is associated with multiple outcomes such as accidents, aggression, risky sex that leads to HIV and school dropout. However, marijuana may also have some therapeutic effects and fourteen states have legalized medical marijuana. Colorado now has an estimated 100,000 medical marijuana patients, and this widespread use raises concerns about marijuana abuse, diversion and other consequences of use. Unfortunately, there are few data describing the impact of medical marijuana legalization. The Specific Aims and hypotheses of this proposed 5-year R01 study are: 1. During Phase I we will determine how the evolving medical marijuana industry operates from the perspective of consumers, MMC owners and prescribing physicians, as well as ascertain the views of drug treatment and health care providers concerning how medical marijuana impacts their responsibilities. As this Phase is designed to gather information to inform Phase III, there are no hypotheses. 2. In Phase II we will conduct an ongoing assessment of the epidemiology of medical marijuana, including change in the number of consumers and MMCs and consumers'and physicians'reasons for medical marijuana. Hypothesis 2: The increase in medical marijuana availability will lead to an increase in the prevalence of marijuana use and marijuana-related problems. 3. In Phase III we will assess the prevalence of marijuana abuse and dependence, diversion, other drug use, general health and health care utilization among medical and non-medical marijuana users. Hypothesis 3a: At baseline, there will be fewer medical marijuana users with cannabis dependence and they will have lower rates of other drug use and health problems than non-medical marijuana users. Hypothesis 3b: Over time, increase in rates of cannabis dependence, other drug use and health problems will be greater for medical marijuana users than non-medical marijuana users. Hypothesis 3c: Among medical marijuana users, there will be an increase in the diversion of medical marijuana over time. 4. Also in Phase III we will assess HIV-related sex risk behaviors among medical and non-medical marijuana users. Hypothesis 4a: At baseline, fewer medical marijuana users will report HIV sex-related risk behaviors than non-medical marijuana users. Hypothesis 4b: Over time, medical marijuana users will increase their engagement in HIV related sex-related risk behaviors more than non-medical marijuana users. PUBLIC HEALTH RELEVANCE: The number of medical marijuana users in Colorado increased 2000% in 2009. This increase raises concerns about abuse, diversion and other health consequences of marijuana use. This study seeks to provide timely information on the medical marijuana system and its public health impact so that policy makers and medical providers can effectively address medical marijuana use.